Edinburgh , Scotland -LRB- CNN -RRB- -- The three main UK party leaders gave a joint promise Tuesday that Scotland will get extra powers if it opts to stay part of the United Kingdom in a landmark vote in two days ' time .

Opinion polls have put the pro-independence and pro-union camps neck-and-neck in the run-up to Thursday 's referendum .

With the survival of the 300-year-old union of England , Scotland and Wales as Great Britain on a knife-edge , UK Prime Minister David Cameron of the Conservatives , coalition partner Nick Clegg of the Liberal Democrats and Labour leader Ed Miliband have joined forces to beg Scotland to stay .

In a pledge published on the front page of Scotland 's Daily Record newspaper , the three leaders say that if Scotland 's voters reject independence , work to devolve `` extensive new powers '' from the central government in Westminster will start Friday .

The issue of spending on social welfare and health care , through the National Health Service , has been central to the pro-independence campaign . Questions over the economy and taxation have also been key .

In their promise , the leaders say `` we can state categorically that the final say on how much is spent on the NHS will be a matter for the Scottish Parliament . ''

They conclude , `` People want to see change . A No vote will deliver faster , safer and better change than separation . ''

` Once-in-a-lifetime opportunity '

With a huge turnout expected for the referendum , both sides are desperate to win over the undecided voters who could hold the future of the United Kingdom in their hands .

In a speech Monday in Aberdeen , Scotland , Cameron made an emotional plea for Scotland not to `` end the United Kingdom as we know it '' and warned that there was no turning back if voters decide on independence .

`` This is a decision that could break up our family of nations , and rip Scotland from the rest of the UK , '' he said .

`` And we must be very clear . There 's no going back from this . No rerun . This is a once-and-for-all decision .

`` If Scotland votes yes , the UK will split , and we will go our separate ways forever . ''

Cameron said he spoke for millions of people across the United Kingdom -- including in Scotland -- who would be `` heartbroken '' to see Scotland leave , and he warned that it would be a `` painful divorce . ''

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The `` Yes Scotland '' campaign , led by Scotland 's First Minister Alex Salmond , says that only by voting for independence can Scotland be sure that it will have full control of its own taxation and spending .

Salmond , speaking in Edinburgh on Monday , called the vote a `` once-in-a-lifetime opportunity '' for Scotland .

`` Nobody goes back '' from independence , he said . `` The evidence is that more and more people in Scotland are wanting to put Scotland 's future into Scotland 's hands . ''

The yes/no referendum question will ask voters , `` Should Scotland be an independent country ? ''

Scotland 's vote : What you need to know

White House : We want a strong , robust UK

White House spokesman Josh Earnest weighed into the debate Monday , echoing remarks previously made by U.S. President Barack Obama .

`` This is a decision for the people of Scotland to make , '' he said . `` We certainly respect the right of individual Scots to make a decision about the -- along these lines .

`` But , you know , as the President himself said , we have an interest in seeing the United Kingdom remain strong , robust , united and an effective partner . ''

Many unanswered questions revolve around defense if Scotland votes for independence . The Scottish National Party , headed by Salmond , has said it wants to remove the UK nuclear submarine fleet from Scottish waters as soon as possible .

' A fool 's errand '

The debate has polarized opinion in the business world , as well as in political circles .

The Financial Times published a full-page editorial last week headlined , `` The case for union is overwhelming . The path of separation is a fool 's errand . ''

Speaking to CNN 's Christiane Amanpour this week , editor Lionel Barber said , `` We really believe that it would be very dangerous and highly risky in terms of economics . ... We believe that the ` Yes ' campaign has not answered important questions -- notably : What will the currency be ? We do n't know . ''

The three main parties in Westminster have said that an independent Scotland would not be able to keep the pound . There are also unanswered questions about tax increases and the fiscal implications of independence , Barber said .

While the newspaper does not deny that Scotland has the right to choose independence , he said , `` We are just pointing to the dangers of uncertainty , the price of uncertainty , which will affect ordinary people . ''

Salmond , who met with business leaders in Edinburgh , said the `` impressive array '' of business leaders who back the independence campaign `` is a very strong point in the ` Yes ' campaign . ''

`` These are the people looking forward to the realization that Scotland can have a more prosperous economy but also wants a more just society . And their voice and that combination of things is something that will carry a good deal of weight off the ballot sheet . ''

CNN 's Max Foster reported from Edinburgh , and Laura Smith-Spark wrote and reported in London .

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Leaders of the three main UK parties give Scotland a joint promise of extra powers

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Voters will go to the polls Thursday to decide on independence for Scotland

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Prime Minister David Cameron begs Scotland to stay , says it 's a `` once-and-for-all decision ''

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Pro-independence chief Alex Salmond says Scottish people want their future in their hands